We have measured electroahsorption (EA) in a-Si:H p-i-n solar cells in order to estimate their built-in potentials Vbr. This method was pioneered by the Osaka University group; in simple cases Vbi is identified with a parameter V0 obtained by a simple linear fit to the dependence of EA on external bias voltage. Using signal averaging techniques we are able to measure V0 with a reproducibility of better than 2%. In one sample from the Institute of Energy Conversion with an a-SiC:H p+ layer we estimate Vbi = 0.97 ± 0.06 V, which is in good agreement with previous JVT measurement. The reason that the error estimate is larger than the reproducibility error is the presence of several small effects not accounted for by the original EA theory: in particular V0depends slightly on the laser wavelength. We attribute these effects to the small portion of Vbidropped in the a-SiC:H p+ layer, which has a different electroabsorption than a-Si:H. We discuss models for the wavelength dependence.